In the Fall of 1973, a study was initiated in Stickney, Illinois, and optimally fluoridated community. Baseline dental examinations were made of 482 children in grade 5 attending parochial and public schools in the community. Participants were randomly divided into two groups. Children in the test group applied in trays an acidulated phosphate-fluoride gel (1.23% F) for ten minutes on 5 consecutive school days, 3 times during the first school year and rinsed weekly with a 0.2% sodium fluoride solution. Children in the control group followed only the mouthrinse procedure with a placebo solution. In the second and third year treatment years, only the mouthrinse procedures were carried out by each group. Follow-up dental exams were made at the end of the 30-month study to determine the effectiveness of the combined caries preventive regimens.